Impedance-Based String Protection for Multi-Stage Capacitor Banks
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Solution Overview
Problem
Conventional capacitor banks face challenges in sensitive protection due to the impact of current and voltage measurement methods, leading to potential misoperations and prolonged downtime when a capacitor element fails, especially in multi-string and multi-stage configurations.
Innovation Solution
A method for sensitive impedance-based string protection that involves obtaining capacitor string and unit design information, determining steady-state voltage and current data, calculating string impedance, and using incremental per-unit impedance calculations to detect failures, thereby minimizing the impact of transients and manufacturing tolerances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional current and voltage sensors are used to measure each phase current for capacitor bank protection, then the protection system can detect capacitor element failures, but the protection sensitivity is impacted and misoperations occur
Solution Approach 1:
The patent changes the measurement parameter from current-based protection to impedance-based protection. By calculating impedance using voltage and current measurements (Z = V/I) and monitoring incremental changes in impedance, the system achieves higher sensitivity to capacitor element failures while avoiding the misoperations associated with conventional current-only protection methods
Solution Approach 2:
The patent introduces impedance calculation as an intermediary parameter between raw voltage/current measurements and protection decisions. The incremental per-unit impedance change serves as a mediator that filters out transient effects and provides a more reliable indicator of actual capacitor element failures
2Quantity of substance
If capacitor banks are configured with multiple strings and stages to provide adequate capacitance, then the capacitive reactive compensation capability is improved, but the complexity of detecting and isolating failed elements increases
Solution Approach 1:
The patent divides the capacitor bank into multiple independently monitorable strings and stages. By calculating impedance for each string and stage separately using the formula Z_string = V_phase / I_string, the system can identify which specific string or stage contains a failed element, reducing the complexity of failure detection and isolation in multi-string multi-stage configurations
Solution Approach 2:
The patent implements continuous monitoring of incremental per-unit impedance changes in each string and stage, providing feedback that enables real-time detection of capacitor element failures. The system calculates the change in impedance from a reference value and triggers protection when the change exceeds a threshold, enabling early detection before failures propagate
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables early detection of capacitor element failures, reduces the risk of misoperations, and minimizes downtime by accurately identifying faults in steady-state conditions, ensuring reliable operation of capacitor banks.
Implementation Method 1
a string impedance is calculated using the measured voltage and current
Data Source
AI summary
A protection and monitoring system, device, and method for an electric power system, including a capacitor bank having multiple strings in each phase, voltage and current measuring devices, and relays to protect this capacitor bank. Each string can have multiple capacitor units and each unit can consist of multiple capacitor elements. The method may include: determining steady state operating condition using the obtained current and voltages, calculating and storing present time impedance value of each string into memory, calculating the string per unit impedance incremental quantity, detecting capacitor element failure based at least in part on this incremental quantity and calculating number of failed capacitor elements for each event, accumulating the number of failed capacitor elements, and performing a protection action when healthy capacitor elements are subject to an overvoltage limit. The method may be inherently immune or otherwise insensitive to capacitor variations due to aging, temperature change, instrument transformers errors, inaccuracy in data acquisition, and inherent manufacturing unbalance.